fix: Fixed the example code error in multimodality doc

[sunyuhan1998]

In the following section of the Multimodality API documentation, the constructor method for UserMessage used in the example code no longer exists:

spring-ai/spring-ai-docs/src/main/antora/modules/ROOT/pages/api/multimodality.adoc

Lines 38 to 50 in f907196

	For most of the multimodal LLMs, the Spring AI code would look something like this:
	[source,java]
	----
	var imageResource = new ClassPathResource("/multimodal.test.png");
	var userMessage = new UserMessage(
	"Explain what do you see in this picture?", // content
	new Media(MimeTypeUtils.IMAGE_PNG, this.imageResource)); // media
	ChatResponse response = chatModel.call(new Prompt(this.userMessage));
	----

The current PR modifies it to use the Builder pattern for construction.

[ilayaperumalg]

@sunyuhan1998 Thanks for the PR fixing the doc.